For ultrasound velocity estimation, the oscillation of the pulsed ultrasound field has been used to estimate the axial velocity component of the structure of interest. The axial component is the component of the velocity vector in the direction of propagation of ultrasound energy from the ultrasound transducer array. Several methods have been proposed in the literature to estimate the lateral components of the velocity vector (perpendicular to the axial component). For 2-D imaging using 1-D transducer arrays, these include speckle tracking, directional beamforming, and transverse oscillation (TO). In directional beamforming, the received signals are focused along the flow direction for a given depth. The signals for two emissions are then cross-correlated, and the shift between them is found. This is a shift in spatial position of the scatterers, and dividing by the time between emissions, thus, directly gives the velocity magnitude. The angle between the emitted beam and the flow direction must be known before the beamformation can be done. The angle could, e.g., be found from the B-mode image as in conventional spectral velocity estimation.
For 2-D velocity vector estimation using the TO approach, an oscillation oriented transverse to that of the ultrasound pulse is introduced in the ultrasound field by applying the same transmit beam as used in conventional axial velocity estimation and adjusting the apodization of the receive aperture in such a way that the whole aperture resembles two point sources. Two point sources separated in space will give rise to two interfering fields, which creates the transverse oscillation. Using the Fraunhofer approximation, the relation between the lateral spatial wavelength and the apodization function is λx=2λzz0/d, where d is the distance between the two peaks in the apodization function, z0 is a depth, and λz is the axial wavelength. In axial velocity estimation, a Hilbert transform is performed to yield two 90° phase shifted signals; the in-phase signal and the quadrature signal. This enables the direction of the flow to be determined. The 90° phase shift in the transverse direction can be accomplished by having two parallel beamformers in receive. The two receive beams are steered, so that the transverse distance between each beam is λx/4, which corresponds to a 90° phase shift in space. Along with these two TO lines, a center line can be beamformed by a third beamformer for conventional axial velocity estimation.
For 3-D velocity vector estimation using the TO approach, 2-D transducer arrays are used to generate the TO field in both lateral dimensions allowing estimation of the velocity vector components in all three dimensions. 3-D velocity vector estimation using multiple crossed-beam ultrasound Doppler velocimetry and speckle tracking have also been proposed in the literature. There is an unresolved need for other approaches to 3-D velocity vector estimation that are applicable to arrays with a reduced number of connections, such as row-column addressed arrays.